1. Field of the Invention
This invention relates to an X-ray tube, and more particularly to an X-ray tube provided with a target radiating the characteristic X-ray of molybdenum.
2. Description of the Prior Art
In mammography that performs X-ray photography of mammae is performed by the use of low energy X-rays from an X-ray tube using a Mo (molybdenum) target that radiates X-rays. These X-rays contains wavelength components of approximately 0.4 to 0.8 angstrom. In this case, the target (anode) acceleration voltage is on the order of 25 to 40 kV. Specifically, to take an X-ray photograph having fewer geometric blurs, the focal point should be as small as possible. On the other hand, to obtain a sufficient amount of X-ray radiation, a tube current of approximately 100 mA or more is usually required, so the focal point becoming larger. In general, an X-ray photograph is taken using X-ray radiation for a relatively long time, such as 1 to 4 seconds, for example. This results in heating electron focal area of the anode target to a high temperature, so that the electron focal area is susceptible to damage upon repeated operation. Namely, the electron focal area frequently exceeds a temperature of 1700.degree. to 1800.degree. C., i.e., the recrystallization temperature of pure Mo. As a result, the metallic crystals of the electron focal area grow large and the surface of the focal area becomes rough. When such thermal fatigue thereof progresses, the amount of X-ray radiation is reduced, and the X-ray radiation quality becomes progressively harder.
Therefore, in order to alleviate damage to the electron focal area of the target, various alloys have been utilized as materials of the electron focal area. For example, the use of a Mo-Hf (hafnium) alloy was disclosed in Japanese Patent Application Laid-open No. 49-45692. Also, the use of an alloy of Mo and one of 25 wt % or less of a metallic element, having an atomic number between 39 and 46, for example, Nb (niobium) was disclosed in Japanese Patent Application Laid-open No. 49-45693. However, there were no notable improvements with these alloys in comparison with a pure Mo target. With pure W (tungsten) or an alloy of R (rhenium) and W used as the material of the electron focal area, desired characteristic X-rays of Mo cannot be obtained. Also, a complex target of pure W or a Re-W alloy has been used as a material for the electron focal area, and the supporting target base thereof was made of a Mo alloy having large heat capacity (disolosed in Japanese Patent laid open No. 60-198045, and British Patent No. 1,121,407). Mo characteristic X-rays could not be obtained with this structure.
When a conventional X-ray tube with a target made of pure Mo was subjected to a forced operation test, corresponding to repetitive operations of a considerably long time. The state of the target surface became deteriorated at the end of the test, as shown in FIGS. 10 and 11. FIG. 10 is a photomicrograph that shows the rotary anode target surface made of pure Mo enlarged to 5 times the actual size. FIG. 11 is a photomicrograph that shows a portion of the electron focal area enlarged to 30 times the actual size. From these photomicrographs, it can be confirmed that the crystals of the electron focal area of the pure Mo target became larger, and experienced a lot of deep cracks. This test was performed under such conditions that the anode acceleration voltage was 40 kV, the tube current was 150 mA, and 4-second electron bombardments were made repeatedly 400 times at 75-second intervals. In addition, another forced operation test was performed under such conditions that the anode acceleration voltage was 40 kV, the tube current was 260 mA, and 1-second bombardments were made repeatedly 5000 times at 50-second intervals. After this test, it was confirmed that the amount of X-ray radiation was reduced to a value of approximately 46% of the initial amount, as shown in the curve L in FIG. 2, showing the comparison among X-ray radiation characteristics of the invention and the prior arts.